1. Field of Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to a method for removing photoresist after a lithographic process.
2. Description of Related Art
In a semiconductor process, lithographic processes are frequently conducted to define photoresist patterns on a wafer, while the photoresist patterns serve as etching or implantation masks for subsequent patterning or implantation process. After the patterning or implantation process, the photoresist remaining on the wafer is usually removed with plasma ashing followed by solvent stripping. The plasma ashing step removes most of the photoresist with O2-based plasma, and the solvent stripping step completely removes the remaining photoresist and polymer with one or more solvents.
As the underlying substrate includes metal layers, however, the metal layers are charged by the ashing plasma, so that some metal element, especially titanium, may be attacked by the stripping solvents because of the well-known galvanic cell effect. For example, FIG. 4 shows a TEM (Tunneling Electron Microscope) picture of an interconnect structure that has experienced the photoresist removal process of the prior art (plasma ashing+solvent stripping). The interconnect structure includes a tungsten via plug 402, a Ti/TiN barrier layer 404 consisting of a titanium layer 406 and a titanium nitride (TiN) layer 408, and an aluminum line 410. The titanium layer 406 is shown white as having been attacked by stripping solvents, while a normal titanium film should be black in such a TEM picture. The attacked titanium layer 406 causes a higher contact resistance between the tungsten via plug 402 and the aluminum line 410, so the performance of the manufactured semiconductor device is lowered because of RC delay effect.